(1) Equipment.—(a) General.—The equipment used with a heavy searchlight consists of four main units: a sound locator, the searchlight, an optical director, and the generator. Beyond the introduction of remote control, little is known of recent developments in German searchlight equipment. Some searchlights of 200-cm, or larger, diameter have been developed, and the sound locator has possibly been improved by the introduction of some form of electrical amplification. The standard heavy searchlight, however, is the 150-cm (60–inch) size. Information from radio-location equipment is almost certainly passed to the searchlights, but the extent and method of its application are unknown.

(b) The ring-trumpet sound locator derives its name from the construction of the four trumpets or horns as a single unit of ring shape. Ordinary stethoscopic listening by two listeners, one for azimuth and one for elevation, is employed. The base length is 135 cm (53.1 inches), giving a theoretical accuracy of about one-half degree. In average weather conditions, the range is about 6,600 yards. The trumpets can be moved through 360° in azimuth and from 0° to 108° in elevation. The "lag calculator" is in the base of the sound locator. Estimated target and sound speeds (the latter based on weather conditions) are set into the lag calculator, which continuously reconstructs the triangle formed by the line of sound reception, the line of sight (present position), and the target course. The azimuth and elevation of the line of sight are shown electrically both at the sound locator and at the searchlight. The sound locator and the searchlight are connected by a cable.

(c) The 150-cm (60-inch) searchlight (fig. 13) has a glass parabolic reflector of 150-cm diameter. The focal length is 650 mm. The high-current-density arc lamp is self-regulating and is fitted in an inverted position in the projector barrel. The light is of 990 million candle power and has a range in favorable weather of 8,800 yards at a height of 13,000 to 16,500 feet. The current consumption is 200 amperes at 77 volts. The projector can be moved in azimuth through 360°, and in elevation from -12° through the vertical to -12° on the other side. The movement of the projector in azimuth is by means of a control arm, which is normally manipulated by hand; its movement in elevation is by means of either of two handwheels, one on the control arm and one on the opposite side of the projector. Electrical receivers for azimuth and elevation show the azimuth and elevation of the line of sight calculated by the sound locator. The beam is exposed and covered by a shutter of Venetian-blind type. More recent models are believed to be equipped with azimuth and elevation driving-motors which can be operated by automatic remote control from the sound locator or from the optical director; the exposing and covering of the beam on these models is controlled from the optical director. It is understood that the driving motors have three or four speeds, 1 degree and/or 1.5, 4, and 16 degrees per second.

Figure 13.—150-cm (60-inch) standard searchlight. (The elevation receiver is on the side of the drum near the extended hand control, and the azimuth receiver is in the rear.)

(d) The optical director consists of a pair of night glasses of ample magnification mounted on a tripod. The director is fitted with an overhead, open sight and with scales showing the azimuth and elevation to which the night glasses are pointing. When employed with remote-control equipment, it is believed that the optical director is located 30 to 45 yards from the searchlight, thus becoming in effect a control station.

(e) The searchlight generator is driven by an 8-cylinder internal-combustion engine which develops 51 horsepower at 1,500 revolutions per minute. The 24-kilowatt generator gives a direct current of 200 amperes at 110 volts at 1,500 revolutions per minute. The cable to the searchlight projector is 220 yards long.

(2) Mobility.—The sound locator, searchlight, and generator are each mounted on a detachable four-wheeled trailer of standard pattern, towed by a truck. Each section or unit therefore requires three trucks for transportation purposes.

(3) Personnel.—The individual searchlight section is composed of 14 individuals with duties as follows:

(4) Communications.—Field telephones are the normal means of communication, each battery having three telephone-erection parties, with sufficient equipment to connect the searchlight sections to platoon headquarters, which, in turn, are connected to battery headquarters. Communications with battalion headquarters are also normally by telephone. Each battery has one small truck equipped with voice radio for communication with the battalion, and two details with pack voice-radio for use within the battery as required.

b. Light Searchlights

(1) Equipment.—(a) General.—The equipment consists of a 60-cm (23.50-inch) searchlight and a generator. This highly mobile and easily handled equipment, designed for use without a sound locator against low-flying targets, appears to have given satisfaction in the limited role for which it was intended. There are no indications that any changes in the design are contemplated.

(b) The 60-cm (23.58-inch) searchlight has a glass parabolic reflector of 60-cm diameter. The focal length is 250 mm. The high-current-density arc lamp is self-regulating and is fitted in an inverted position in the projector barrel. The light is of 135 million candle power and has a range (in focus) in favorable weather of 5,700 yards at a height of 5,000 feet; with dispersed beam the range is 3,500 yards. The current consumption is 90 amperes at 60 volts. The projector is moved in azimuth and elevation by handwheels operated by the searchlight controller, who is seated behind the projector barrel. The beam is exposed and covered by a shutter of Venetian-blind type.

(c) An 8-kilowatt searchlight generator gives the required current at 85 volts. It is connected to the searchlight by a cable 110 yards long.

(2) Mobility.—The projector is mounted on a detachable two-wheeled trailer, towed by a truck. The same truck carries the generator, which can either be operated in the body of the truck or be unloaded on the ground.

(3) Personnel.—The individual searchlight section is composed of five individuals with duties as follows:

No. 1 lays the searchlight as ordered, and puts the light into action with a dispersed beam. The section commander gives directional orders and orders a search, if required. The search is carried out in S-shape light-tracks across the target course. If No. 1 gets on target, No. 2 puts the beam in focus. No. 1 shuts off the beam on the section commander's orders.

(4) Communications.—Since light searchlights normally operate directly with light-gun platoons, the light-searchlight section from the communication point of view is normally serviced by the light Flak battery or platoon with which the light-searchlight section is operating.